1. Technical Field
This invention applies to rotor blades in general, and to apparatus for damping vibration within a rotor blade in particular.
2. Background Information
Turbine and compressor sections within an axial flow turbine engine generally include a rotor assembly comprising a rotating disc and a plurality of rotor blades circumferentially disposed around the disk. Each rotor blade includes a root, an airfoil, and a platform positioned in the transition area between the root and the airfoil. The roots of the blades are received in complementary shaped recesses within the disk. The platforms of the blades extend laterally outward and collectively form a flow path for fluid passing through the rotor stage. The forward edge of each blade is generally referred to as the leading edge and the aft edge as the trailing edge. Forward is defined as being upstream of aft in the gas flow through the engine.
During operation, blades may be excited into vibration by a number of different forcing functions. Variations in gas temperature, pressure, and/or density, for example, can excite vibrations throughout the rotor assembly, especially within the blade airfoils. Gas exiting upstream turbine and/or compressor sections in a periodic, or "pulsating", manner can also excite undesirable vibrations. Left unchecked, vibration can cause blades to fatigue prematurely and consequently decrease the life cycle of the blades.
It is known that friction between a damper and a blade may be used as a means to damp vibrational motion of a blade. How much vibrational motion may be damped depends upon the magnitude of the frictional force between two surfaces. Frictional force depends upon the amount of surface area in contact between the two surfaces, the frictional coefficients of the two surfaces, and the normal force keeping the surfaces in contact with each other. If the spring rate of the damper (i.e., the normal force) decreases because of fatigue in the spring and/or the thermal environment, the amount of vibrational motion that may be damped similarly decreases. If the surface against which the damper acts decreases in area or wears away from the damper, the effectiveness of the damper is also negatively effected.
Frictional dampers may be attached to an external surface of a blade airfoil, or inserted internally through the airfoil inlet area. A disadvantage of adding a frictional damper to an external surface is that the damper is exposed to the harsh, corrosive environment within the engine. As soon as the damper begins to corrode, its effectiveness is compromised. In addition, if the damper separates from the airfoil because of corrosion, the damper could cause foreign object damage downstream. A damper can be protected from the harsh environment by enclosing it in an external pocket. In most cases, however, the damper must be biased between the pocket and the pocket lid and the effectiveness of the damper will decrease as the damper frictionally wears within the pocket.
In short, what is needed is a rotor blade having a vibration damping device which is effective in damping vibrations within the blade and which minimizes reliance on the spring rate of the damper and the surface are against which the damper acts.